1. Field of the Invention
The present invention relates to a paste used for forming a resistor such as a thick film chip resistor and a hybrid IC, in particular, a resistance paste that does not contains lead, and a resistor formed from the resistance paste.
2. Description of Related Art
Hitherto, as a method of forming a resistor film of an electronic component, a thick film method that uses a resistance paste and a thin film method where a film-forming material is sputtered are well known. Of these, in the thick film method, a resistance paste is printed on a ceramic substrate, followed by sintering to form a thick film resistor. The method, being cheap in equipment and high in the productivity, is widely used in the manufacture of a chip resistor and a resistor of a hybrid IC or the like.
A resistance paste that is used in the thick film method is substantially constituted of electrically conductive particles, glass frit and an organic vehicle that makes these paste-like one suitable for printing. As the electrically conductive particles, ruthenium dioxide (RuO2) and pyrochlore type ruthenium based oxide (Pb2Ru2O7−x, Bi2Ru2O7) are generally used. The reason why the Ru based oxide is used as the electrically conductive particle is mainly because a resistance value smoothly varies with a concentration of the electrically conductive particles.
Furthermore, the glass frit controls a resistance value at an arbitrary value depending on a ratio thereof to the electrically conductive particles and contributes as well to retention of the film strength of the resistor and to the adhesion with the substrate. As the glass frit, lead borosilicate based glass containing much lead such as lead borosilicate glass (PbO—SiO2—B2O3) or lead aluminoborosilicate glass (PbO—SiO2—B2O3—Al2O3) is used. The reason why the lead borosilicate based glass is used as the glass frit is in that the wettability with the Ru based oxide is excellent, the thermal expansion coefficient is close to that of the substrate and the viscosity at the sintering is preferable.
As other electrically conductive particle than the Ru based oxide, iridium dioxide (IrO2) is known as well (JP-B-54-1917). However, the iridium dioxide as the electrically conductive particle has been as well used in a resistance paste that uses glass that contains lead such as the lead borosilicate based glass as the glass frit.
On the other hand, recently, from a viewpoint of environmental protection, lead-free electronic components are advanced. Accordingly, the resistance paste is as well strongly demanded to be free from lead. However, in a combination of electrically conductive particles of the Ru based oxide and the glass frit that does not contain lead, it is known that as a ratio of the electrically conductive particles becomes smaller, the resistance value rapidly goes up (J. Am. Ceram. Soc., 83(10), (2000), p2441-2448). Accordingly, in a thick film resistor formed with the resistance paste, there are problems in that the dispersion of the resistance values at high resistance becomes larger and current noise as well becomes larger.